Electron detector

ABSTRACT

A device for measuring electron densities at a given energy level in an electron beam or the like having strong background noise, for example, in the detection of Auger electric energy spectrums. An electron analyzer passes electrons at the given energy level and at the same time or ad seriatum electrons of at least one adjacent energy level. Detecting means associated therewith produce signals indicative of the densities of the electrons at each energy level and combine these signals to produce a signal indicative of the density of the electrons of the given energy level absent background noise.

United States Patent Hashimoto et al.

1451 Oct. 21, 1975 ELECTRON DETECTOR 3,723,713 3/1973 Banner et al.250/282 Inventors: Hiroshi HaShimom; Akinori 3,833,811 9/1974 Kolke etal. 250/306 Mogaml both of Aklshlma Japan Primary Examiner.lames W.Lawrence [73] Assignee: Nihon Denshi Kabushiki Kaisha, AssistantExaminer-D. C. Nelms Tokyo, Japan Attorney, Agent, or FirmWebb, Burden,Robinson & 22 Filed: Mar. 6, 1974 Webb [21] Appl. No.: 448,480 [57]ABSTRACT A device for measuring electron densities at a given [30]Foreign Application Priority Data energy level in an electron beam orthe like having Mar. 12, 1973 Japan 48-28724 Strong background noise,for example in the detection of Auger electric energy spectrums. Anelectron ana- 52 U.S. c1 250/305; 250/282 lyzer P e electrons t thegive" energy level and at 51 Int. (:1. H01J 39/00 e Same ad Senammelectrons of one 58 Field of Search 250/305, 306, 440, 282 adtacerft w eD fl means aswqlated therewtth produce slgnals mdicatwetof the densitlesof [56] References Cited the elletitrons gt each ene1l"g y dlevetlandfctcg'lmtziine tthes? s1gna s 0 pro uce a stgna 1n tca we 0 e ensl y0 UNITED STATES PATENTS the electrons of the given energy level absentback- 3,626,184 12/1971 Crewe 250/305 ground noise 3,631,238 l2/l97lMacDonald 250/305 3,718,818 2/1973 Arx et al 250/305 8 Claims, 21Drawing Figures MODULATION VOLTAGE ADDING 1a GEN ClRCUI'r 12c. VOLTAGE JSol/RC a 10 g 11 is; 1:0 l 17 GATE PULSE 'lNTEfi-RA- cmcul-r COUNTER g?"SUBTM I A M P. CTID V 1 L 9 16 2] CIRCUIT I G-ATE PULSE INTEGRA- 14cmcutT COUNTER I SUB TRA- c T10 CIRCUIT INTEG'RA- INTEcTRA- PULSECOUNTER PULSE COUNTER Sheet 1 of 6 GATE cmeurr GATE. cmcurr VOLTAGEG-EN.

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6] 62 68" PULSE COUNTER T AMP. CISCUT eeo"? SQUARE 35233 213 f izzhll[d] W q L L Ll H HILH ELECTRON DETECTOR This invention relates to ananalyzing device for measuring weak signals detected along with a largeamount of background noise.

To investigate substances existing in the thin layer immediately belowthe specimen surface, the utilization of the so-called Auger electronenergy spectrum, obtained by irradiating the specimen with primaryelectrons or X-rays, is most desirable. However, with conventional typeenergy devices Auger electrons are detected or analyzed along withsecondary (or photo) electrons and backscattered electrons, all of whichare ejected from the specimen simultaneously.

Moreover, the density of the Auger electron energy spectrum is muchweaker than that of the photo electron energy spectrum in that theformer spectrum embodies a considerable amount of background noise, thusmaking it difficult to accurately measure the Auger electron energyspectrum with the apparatus presently available.

In an attempt to overcome this problem, a method was devised andincorporated in some conventional type analyzing devices whereby theoutput signal of the electron energy analyzer is differentiated, saiddifferentiation being based on the fact that the change in strength ofthe background noise is much smaller than that of the Auger electrons.However, in this method, the detected input signal must be sufficientlystrong to provide an analogue input signal for the lock-in amplifier.Accordingly, especially when the analyzer output signal is pulsed,intergration must be carried out for some time which makes Augerelectron energy measurement quite time consuming. As a result, it isimpos' sible to observe rapid changes in the specimen with time; forexample, when the specimen temperature is increased linearly with time.Another drawback with this method is that, since only the change rate ofthe input signal is measured, the measurement accuracy of the Augerelectron density is not of a high order and thus, it is impossible toanalyze substances lying immediately below the specimen surfacequantitatively.

Accordingly, an advantage of this invention is to provide an analyzingdevice for measuring the density of the Auger electron energy spectrumpeaks more pre cisely.

Another advantage of this invention is to provide an analyzing devicecapable of observing the change in the Auger electron spectrum peakheight when the specimen temperature is varied with time.

A still further advantage of this invention is to provide an analyzingdevice for precisely measuring the density of the Auger electron energyspectrum peaks even when the detected signal is pulsed.

Briefly according to this invention, a typical analyzer device isprovided with additional means for detecting a plurality of signalsobtained by the electron energy analyzing means, such that said signalsrespectively correspond to the successive values of the electron beamenergies and a function of at least one of said signals is subtractedfrom one of the other said signals by a processing circuit network.

Additional features and advantages of this invention will become morereadily apparent by reading through the following detailed descriptionin conjunction with the accompanying drawings of which:

FIG. 1 is a block schematic showing one embodiment of the analyzingdevice according to this invention,

FIGS. 2 and 3 are diagrams for explaining the operation of theembodiment shown in FIG. 1,

FIG. 4 is a block schematic showing one embodiment of the analyzingdevice according to this invention,

FIGS. 5(a), (b), (c) and (d) are diagrams for explaining the operationof the embodiment shown in FIG. 4,

FIGS. 6 and 7 are schematics showing the embodiments according to thisinvention,

FIG. 8 is a diagram for explaining the operation of the embodiment shownin FIG. 7,

FIGS. 9 and 11 are schematics showing the embodiments according to thisinvention, and

FIGS. 10 and 12 are diagrams for explaining the operations of theembodiments shown in FIGs. 9 and 11 respectively.

Referring to FIG. 1, an electron energy analyzer section 1 incorporatesan electron gun 2 for irradiating a specimen 3 with a primary electronbeam 4. Two electrostatic electrodes 50 and 5b separate electrons 6emanating from the specimen 3 according to energy. Electrons 6 includingAuger electrons and secondary electrons emanating from said specimenpass through an input slit 7 of the electrode 5b and are subjected tothe electric field existing between the electrodes 5a and 5b. Thus, thetrajectory of the electrons is determined according to the energy ofelectrons so that only electrons having an energy properly correspondingto the electric field strength are able to pass through the output slit8 for subsequent detection by an electron detector 9.

An adding circuit 10 determines the electric field strength, the outputterminals of said adding circuit being connected to the two electrodes5a and 5b. A DC. voltage source 11 and a modulation voltage generator 12are connected to the input terminals of the adding circuit 10. Arecorder 13 records the output voltage of the DC. voltage source 11together with the processed output signal of the analyzer section 1.

FIG. 2(a) shows the output of the adding circuit 10 during a micro lapseof time. In the figure, the modulation voltage, i.e. the output of themodulation voltage generator 12, has a width e. The output voltage E ofthe DC. voltage source 11 varies at a much slower rate than the outputof the generator 12. Timing signals, t, to are supplied by the generator12 to a circuit network 14.

In the embodiment shown in FIG. 1, since the electrons detected by thedetector 9 are few in number, the detector output signal is pulsed.Accordingly, the circuit network 14, in this case, incorporates twopulse counters l5 and 16. The processing sequence is as follows. Theoutput signal of detector 9, after being amplified by an amplifier l7,enters pulse counters 15 and 16 via gate circuits l8 and 19. The twopulsed signal components, after being pulse counted, then enterintegration circuits 20 and 21 the respective outputs of which aresubtracted by a subtraction circuit 22 prior to being fed into therecorder 13. In this case, the subtraction circuit 22 generates zerosignal when the subtraction value becomes minus.

FIG. 2(b) shows the energy distribution of the electrons detected by thedetector 9. The ordinate indicates the density of the detected electronsand the abscissa indicates the energy of the detected electrons which isproportional to the output voltage of the adding circuit 10. As shownfor the particular position during sweep shown in FIG. 2b, the outputvoltage corresponding to peak P equals E and the modulation voltagewidth e includes peak P, the peak spread equalling E (e/4) and E (e/4).The background noise level near peak P is shown as E (e/Z) and E (e/2),the outer limits of the modulated signal for the given sweep position.

Timing signals generated by modulation generator 12 control the gatecircuit 18 so as to pass the input signal during time intervals T, and TAccordingly, the output of the integration circuit 20 is as shown inFIG. 2(a). The final counting value Nn shown in FIG. 2(c) corresponds tothe intensity background noise surrounding peak P. Moreover, when thebackground sig nal varies linearly as shown in the figure, Nn alsocorresponds to the density of the background signal component of thespread of peak P.

The remaining timing signals generated by generator 12 control the gatecircuit 19 so as to pass the input signal during the time intervals Tand T Accordingly, the output of the integration circuit 21 is as shownin FIG. 2(d). In this case, the final counting value Np+n corresponds tothe density of peak P plus that of the background signal componentincluded in the peak spread.

Incidentally, the t timing signal operates the subtraction circuit 22,and more or less simultaneously, resets integration circuits and 21.

It will be apparent from the above that the output of the subtractioncircuit 22, which is recorded by the recorder 13, represents the densitycorresponding to the peak signal itself.

FIGS. 3(a), (b), (c) and (d) are graphical illustrations for explainingan operation mode of the embodiment shown in FIG. 1 which differs fromthat heretofore described. In this case, the generator timing signalsare generated at intervals t t t and t as shown in FIG. 3(a), where timeinterval T equals the sum of time intervals T and T In this mode, thegate circuit 18 is controlled so as to pass the input signal during timeintervals T and T while the gate circuit 19 is controlled so as to passthe input signal during time interval T Accordingly, the output of theintegration circuit 20 appears as shown in FIG. 3(b) and that of theintegration circuit 21 appears as shown in FIG. 3(c). In other respects,this operation mode is identical to the former as explained in theaforegoing. Although the timing pattern of the generated signals in FIG.3(a) is simpler than that in FIG. 2(a), peak measurement accuracy is notas good. However, when background noise surrounding the peak is almostconstant or varies very gradually as shown in FIG. 3(d), it is possibleto obtain sufficient accuracy to ensure effective peak measurement.

FIG. 4 is a block schematic showing another embodiment of the analyzingdevice according to this invention in which the circuit network 23 isdifferent from that described in FIG. 1. In this embodiment, themodulation voltage generator 12 generates tming signals at instances tt,, t t, and t as'shown in FIG. 2(a), so as to control gate circuits 18and 19. The output (shown in FIG. 5(a) of the gate circuit 18 issupplied the reversible counter 24 becomes zero or the number of pulsesfed into terminal b excess the number (Nn Nn of pulses fed into terminala, the counter 24 generates signals as shown in FIG. 5(c), said signalsbeing fed into the gate circuit 25 so as to open it. As a result, theoutput of the gate circuit 19 is fed to a counter circuit 26 via thegate circuit 25 only during the time when the pulses passing through thegate circuit 19 are much larger than the pulses passing through the gatecircuit 18. Accordingly, the output signals shown in FIG. 5(d)correspond to the peak density value itself as in the case of the priorembodiment.

FIG. 7 is a block schematic showing another embodiment of the analyzingdevice according to this invention. In this embodiment, the modulationvoltage generator 12 has been dispensed with and three electrondetectors 34, 35, and 36 are used instead of one. In an analyzer section37, the three detectors are arranged adjacently and below three slits38, 39 and 40. Electrons having slightly different energies are focussedby the electric field existing between the electrodes 5a and 5b so as topass through said three slits. The electron trajectories 41, 42 and 43are determined by the output of the DC voltage source 11. Accordingly,the position of the slits corresponds to the energy of the electronsfocussed at the slits.

In FIG. 8, the abscissa indicates the slit position and the ordinateindicate the density of the electrons passing through the respectiveslits. Electrons corresponding to the spectrum peak P are focussed tothe center of the slit 39 having a slit width d, and electronscorresponding to the background noise enveloping the spectrum peak P arefocussed at the center of slits 38 and 40 each having a slit width(d/2). Accordingly, when the background noise component contained in thespectrum varies linearly as shown in FIG. 8, the net density of thespectrum peak P is obtained by subtracting the sum of the outputs of thedetectors 34 and 36 from the output of the detectors 35.

This subtracting process is effected by the circuit network 44. Theanalogue output of the detectors 34 and 36 are fed into the input of theadding circuit 45 via amplifiers 46 and 47, and the added output of theadding circuit 45 is fed into the input terminal b of the subtractioncircuit 48. At the same time, the analogue output of the detector 35 isfed into the input terminal a of the subtraction circuit 48 viaamplifier 49. After subtraction (a-b), the output signal of thesubtraction circuit 48 is fed into the recorder 13 and recorded togetherwith the output signal of the DC. voltage source 11 which is variedcontinuously. In this case, the subtraction circuit 48 generates zerosignal when the subtraction count value (a-b) becomes minus. Thus, thenet energy spectrum peaks of the electrons emanating from the specimenare recorded.

FIG. 9 is a block schematic showing another embodiment of the analyzingdevice according to the invention. In analyzer section 50 of thisembodiment, the three detectors 34, 35 and 36 are characterized in thattheir outputs are pulsed and the three slits 51, 52 and 53 are of thesame width. Accordingly, the construction of the circuit-network 54 isdifferent from that of the previous embodiments. The output ofamplifiers 46 and 47 are as shown in FIG. 10(b) and (0) respectively,and both outputs are fed into the same input terminal of a flip-flopcircuit 55 in the circuit network 54. The

number of output pulses of the circuit 55 is equal to the mean number ofthe outputpulses of the amplifiers 46 and 47 as shown in FIG. l0(d).-Theoutput of the circuit 55 is fed into the input terminal b of a squarepulse generator 56. And another input terminal a of the gen erator 56 isfed with the output pulses (shown in FIG. (a) of the amplifier 48 viadelay circuit 57 which slightly delays its input pulses. The squarepulse generator 56 generates square pulses as shown in FIG. 10(e) byusing the input pulses from terminal a as rise signals of square pulsesand by using the input pulses from terminal b as fall signals of squarepulses. And a gate circuit 58 iscontrolled so as to pass the inputpulses only during the time when thesquare pulsessare fed from thegenerator 56. As a result, the number of input pulses fed into a pulsecounter 59 equals the difference be tween the pulse number from thedetector 35 and the mean pulse number from the'detectors 34 and 36.Namely the output of the pulse counter 59 corresponds to the net energyspectrum peak strength of the electron beam. And this output is fedintothe brightness control grid of the cathode-ray tube 32.

Additionally, the specimen 3 is scanned with the electron beam 4 bymeans of scanning coils 29X and 29Y complete with their signal generator30 as same as the embodiment shown inv FIG. 6; Consequently, thescanning image of Auger electrons having specific energy is displayed onthe screen of'the cathode-ray tube FIG. 11 is a block schematic showingthe other embodiment according to the inventionrThis embodiment shown inF lg. 9 in that number of the detectors is two in analyzer section 60.This embodiment having two detectors is effective only when the energyspectrum background noise of the electrons is constant or nearlyconstant. I a t In an analyzer section 60, two electron detectors 61 and62 are arranged adjacently and'below two slits 63 and 64' each havingsame slit width. The output pulses of the detectors'6l' and 62 areamplified by amplifiers 65' and 66 and'processed by a circuit network 67so as to subtract the pulses of the output of the amplifier 66 from thatof the amplifier 65. Output of the amplifier 65, is split and one outputis fed into the gate circuit 58 and another output is fed into the inputterminal a of the square pulse generator 56 via delay circuit 57 whichslightly delays its input signal: Input pulses of the gate circuit 58are as shown in FIG. 12(0). The output of the amplifier 66 is shown inFIG. 12(b) and is fed to the input terminal b of the generator 56. Thesquare pulse generator 56 generates square pulses, as shown in FIG.12(c), using input pulses of the tenninal a as rise signals of thesquare pulses and by using input pulses of the terminal b as fallsignals of the square pulses. And the gate circuit 58 is controlled soas to pass the input pulses only during time when the square pulses arefed from the generator 56. Accordingly, the number of input pulses of apulse counter 68 is equal to the difference between the number of outputpulses of the detector 61 and that of the detector 62'as shown in FIG.

FIG. 6 isa'block schematic showing another embodiment of the analyzingdevice according to this invention. In the analyzer section 27 of theembodiment, the primary electron beam 4 is focussed on the surface of la specimen 3 bya condenser lens 28 and is made to scanning signalgenerator 30. The detector 9, in this case, is designed to detectelectron density directly. Accordingly,'the circuit network 31 does notrequire any counting circuits.

5 During the operation of the analyzing device accord- 10 being thecase, the output of the detector 9 depends on the position at which thespecimen surface is irradiated by the primary electron beam. The outputsignal of the detector 9, thus dependent, is amplified by the amplifierl7 and processed by the circuit network 31. After l5 which, the signalis supplied to the brightness control grid of a cathode-ray tube 32.Deflection coils33X and 33Y are'energized by the generator 30 which alsoenergizes scanning coils 29X and 29Y as previously mentioned. In thisway, a scanning Auger electron image is displayed on the screen of thecathode-ray tube 32.

l. A device for detecting signals indicative of the density of electronsof given energy level absent background noise in an electron beam havinghigh background noise for all energy levels comprising:

i a. analyzing means for discretely passing electrons of .the givenenergy level and at least one different ad- I jacent energy level, v

b. detecting and integrating means associated with said analyzer fordetecting the electrons passed by said analyzer and producing a firstsignal indicative of the density of electrons of the given energy leveland at least one second signal indicative of the density of theelectrons of at least one different adjacent energy level, I I c.processing means for subtracting a function of the at least one secondsignal from the firstsignal to provide a signal indicative of thedensity of electrons .of the given energy level absent background noise.p

2. A device according to claim 1 in which the conditions of theanalyzing means are swept such that an electron energy spectrum isproduced. I

,3. A device for detecting an electron energy spectrum of an electronbeam having strong background noise at all energy levels comprising:

a. analyzing means for spatially dispersing and separately focussingelectrons according to their re-.

spective energy levels and passing electrons of one energy level at atime,

' b. control means for modulating the conditions of the analyzer with analternating signal periodically varying the energy level of theelectrons passed by the analyzer about a detecting energy level which isslowly and continuously swept,

c. detection and integration means associated with said analyzing'meansand synchronized with said alternating signal for detecting electronspassed by said analyzer producing a first signal indicative of thedensity of electrons of the detection energy level and producing atleast one second signal indicative of the density of the electrons in atleast one different adjacent energy level,

d. processing means for subtracting a function of at least one secondsignal from the first signal to provide a signal indicative of thedensity of the electrons at the detection level absent background noiseand thereby producing a spectrum as the detection level is swept.

4. A device according to claim 3 in which the control means modulatesthe analyzer symmetrically about the detection level and the detectionand integration means detects a first signal indicative of the densityof the electron level during two periods including the instances inwhich the analyzer passes electrons at the detection level and detectssecond signals during periods when the first signals are not beingdetected.

5. A device for displaying a scanning image of the Auger electronscomprising:

a. means for irradiating a specimen with a focussed primary electronbeam so as to cause the specimen to eject Anger and other kinds ofelectrons from the specimen,

,b. scanning means for scanning said primary electron beam over thespecimen,

c. analyzing means for spatially separating and separately focussing theelectrons according to their respective energies,

d. means for periodically detecting the output of the said analyzingmeans during a background noise detection period and during a backgroundnoise and spectrum signal detection periods, said detection means beingsynchronized with said control means,

e. subtraction means for subtracting output of said analyzing meansduring the background noise detection period from the output of saidanalyzing means during the background noise and spectrum signaldetection period, and

f. display means for displaying a scanning image brightness modulated bythe output signal of said subtraction means, said display means beingsynchronized with the scanning means.

6. A device for detecting a signal indicative of the density ofelectrons of given energy level absent background noise in an electronbeam having high background noise for all energy levels comprising:

a. analyzing means for spatially separating and separately focussing theelectrons according to their respective energies,

b. detecting means comprising a plurality of detectors for producing asignal indicative of the density of the electrons focussed by saidanalyzing means on said detectors, said detectors being locatedadjacently such that their signals are indicative of the densities ofthe electrons at adjacent energy levels, and

c. processing means for subtracting a function of the signal indicativeof the density of electrons at adjacent energy levels from the signalindicative of the given energy level to provide a signal indicative ofthe density of electrons of the given energy level absent backgroundnoise.

7. A device according to claim 6 in which the detecting means comprisesthree adjacent detectors for detecting different adjacent energy levelsin which the processing means subtracts a function of the signal fromthe detectors detecting the high and low energy levels from the signalof the other detector.

8. A device for displaying a scanning image of the Auger electrons of agiven energy level comprising:

a. means for irradiating a specimen with a focussed primary electronbeam causing the specimen to eject Auger and other kinds of electronsfrom the specimen,

b. scanning means for scanning said primary electron beam over thespecimen,

c. analyzing means for spatially dispersing and separately focussing theelectrons dislodged from the specimen according to their respectiveenergy levels,

d. detecting and integrating means comprising a plurality of adjacentdetectors for detecting the electrons focussed by said analyzing meansproviding signals indicative of the density of the electrons of thegiven and adjacent energy levels,

e. processing means for subtracting a function of the signals indicativeof the given energy levels from a signal indicative of the adjacentenergy levels to provide a signal indicative of the electron density ofelectrons of the given energy level absent background noise,

f. display means synchronized with said scanning means for displayingthe scanning image brightness modulated by the output of said processingmeans. 45

1. A device for detecting signals indicative of the density of electronsof given energy level absent background noise in an electron beam havinghigh background noise for all energy levels comprising: a. analyzingmeans for discretely passing electrons of the given energy level and atleast one different adjacent energy level, b. detecting and integratingmeans associated with said analyzer for detecting the electrons passedby said analyzer and producing a first signal indicative of the densityof electrons of the given energy level and at least one second signalindicative of the density of the electrons of at least one differentadjacent energy level, c. processing means for subtracting a function ofthe at least one second signal from the first signal to provide a signalindicative of the density of electrons of the given energy level absentbackground noise.
 2. A device according to claim 1 in which theconditions of the analyzing means are swept such that an electron energyspectrum is produced.
 3. A device for detecting an electron energyspectrum of an electron beam having strong background noise at allenergy levels comprising: a. analyzing means for spatially dispersingand separately focussing electrons according to their respective energylevels and passing electrons of one energy level at a time, b. controlmeans for modulating the conditions of the analyzer with an alternatingsignal periodically varying the energy level of the electrons passed bythe analyzer about a detecting energy level which is slowly andcontinuously swept, c. detection and integration means associated withsaid analyzing means and synchronized with said alternating signal fordetecting electrons passed by said analyzer producing a first signalindicative of the density of electrons of the detection energy level andproducing at least one second signal indicative of the density of theelectrons in at least one different adjacent energy level, d. processingmeans for subtracting a function of at least one second signal from thefirst signal to provide a signal indicative of the density of theelectrons at the detection level absent background noise and therebyproducing a spectrum as the detection level is swept.
 4. A deviceaccording to claim 3 in which the control means modulates the analyzersymmetrically about the detection level and the detection andintegration means detects a first signal indicative of the density ofthe electron level during two periods including the instances in whichthe analyzer passes electrons at the detection level and detects secondsignals during periods when the first signals are not being detected. 5.A device for displaying a scanning image of the Auger electronscomprising: a. means for irradiating a specimen with a focussed primaryelectron beam so as to cause the specimen to eject Auger and other kindsof electrons from the specimen, b. scanning means for scanning saidprimary electron beam over the specimen, c. analyzing means forspatially separating and separately focussing the electrons according totheir respective energies, d. means for periodically detecting theoutput of the said analyzing means during a background noise detectionperiod and during a background noise and spectrum signal detectionperiods, said detection means being synchronized with said controlmeans, e. subtraction means for subtracting output of said analyzingmeans during the background noise detection period from the output ofsaid analyzing means during the background noise and spectrum signaldetection period, and f. display means for displaying a scanning imagebrightness modulated by the output signal of said subtraction means,said display means being synchronized with the Scanning means.
 6. Adevice for detecting a signal indicative of the density of electrons ofgiven energy level absent background noise in an electron beam havinghigh background noise for all energy levels comprising: a. analyzingmeans for spatially separating and separately focussing the electronsaccording to their respective energies, b. detecting means comprising aplurality of detectors for producing a signal indicative of the densityof the electrons focussed by said analyzing means on said detectors,said detectors being located adjacently such that their signals areindicative of the densities of the electrons at adjacent energy levels,and c. processing means for subtracting a function of the signalindicative of the density of electrons at adjacent energy levels fromthe signal indicative of the given energy level to provide a signalindicative of the density of electrons of the given energy level absentbackground noise.
 7. A device according to claim 6 in which thedetecting means comprises three adjacent detectors for detectingdifferent adjacent energy levels in which the processing means subtractsa function of the signal from the detectors detecting the high and lowenergy levels from the signal of the other detector.
 8. A device fordisplaying a scanning image of the Auger electrons of a given energylevel comprising: a. means for irradiating a specimen with a focussedprimary electron beam causing the specimen to eject Auger and otherkinds of electrons from the specimen, b. scanning means for scanningsaid primary electron beam over the specimen, c. analyzing means forspatially dispersing and separately focussing the electrons dislodgedfrom the specimen according to their respective energy levels, d.detecting and integrating means comprising a plurality of adjacentdetectors for detecting the electrons focussed by said analyzing meansproviding signals indicative of the density of the electrons of thegiven and adjacent energy levels, e. processing means for subtracting afunction of the signals indicative of the given energy levels from asignal indicative of the adjacent energy levels to provide a signalindicative of the electron density of electrons of the given energylevel absent background noise, f. display means synchronized with saidscanning means for displaying the scanning image brightness modulated bythe output of said processing means.